Wind turbine blade with tripping device and method thereof

ABSTRACT

A wind turbine and a method of reducing extreme loads and fatigue loads on a wind turbine by using a passively activated tripping device arranged on the pressure side of the wind turbine blade. The tripping device is configured to interrupt the passing airflow, when activated, and transform the airflow into a turbulent airflow. The tripping device is positioned close to the leading edge, wherein its dimensions are optimized so that it reduces the maximum lift coefficient as well as increases the minimum lift coefficient which in turn reduces the range of the lift coefficient. The tripping device is activated at both negative and positive angle-of-attacks outside the normal operating range.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method and a wind turbine with a windturbine blade having one or more passively activated tripping devicesarranged on a side surface near the leading edge of the wind turbineblade. The tripping device is configured to interrupt the laminarairflow passing over the wind turbine blade in the chord-wise direction.The tripping device is designed to reduce extreme and fatigue loadsexperienced in the wind turbine without affecting the aerodynamicperformance within the normal operating range.

Description of Related Art

A main challenge when designing wind turbine blades is to design a windturbine blade that is structurally robust and has an aerodynamic profilecapable of operating under conditions where the surface is clean as wellas more rough, e.g., due to deposits from insects and dirt. One way ofsolving this problem is to provide a wind turbine blade having aroughness insensitive aerodynamic profile. However, such an aerodynamicprofile will have a large maximum lift coefficient which in turnincreases the extreme loads and fatigue loads experienced in the windturbine blade and other components in the wind turbine.

Another solution is to outfit the aerodynamic profile of the windturbine blade with vortex generators, slats or Gurney flaps at theleading or trailing edges to increase the lift coefficient and/or theangle-of-attack in order to compensate for the lift loss due to thesurface roughness. However, such passively activated devices alsoincreases the extreme loads and fatigue loads in the wind turbine bladeand other components in the wind turbine. It is also known that it ispossible to add actively controlled deformable units to the aerodynamicprofiles capable of altering their outer profile based on controlsignals received from a controller. However, such active units introduceweak spots in the wind turbine blade and will result in a less reliantsolution compared to the above-mentioned passive devices.

U.S. Pat. No. 8,602,739 discloses a wind turbine blade outfitted with astall strip arranged on the pressure side for introducing stall, andthus reducing the lift. This stall strip is specifically designed forreducing the minimum lift coefficient at negative angle-of-attacks. Thestall strip has a scalene triangular shape and is placed at a chordlength of 5% to 30% from the leading edge for optimal stall effect. U.S.Pat. No. 8,602,739 also states that this stall strip can have anisosceles triangular shape and can be placed at a chord length of 2% to5%, however, this will result in a reduction in the aerodynamicperformance of the wind turbine blade and, thus, in the power productionin the normal operating range.

The U.S. Pat. No. 8,083,491 discloses a wind turbine blade outfittedwith a stall strip or a deformable unit connected to actuator meanswhich is placed at a chord length of 1% to 20% from the leading edge.This stall strip or deformable unit is specifically designed forreducing the minimum lift coefficient at negative angle-of-attacks.However, this stall strip or deformable unit will also increase the dragand, thus, reduce the aerodynamic performance during normal operation.

The U.S. Pat. No. 8,192,161 discloses a wind turbine blade with adynamic stall regulation system which is actively operated by anintegrated drive unit. A controller uses a load sensor to determine whenthe loading exceeds a threshold which, in turn, triggers the activationof the drive unit which extends the respective deflector element. Thisadds to the complexity and total costs of such a system. The extendabledeflector elements are designed to separate the airflow passing over thewind turbine blade when deployed, thereby increasing the drag andreducing the lift. The air deflectors thus act as stall devices forminga separation point where the boundary layers of the passing airflow areseparated from the pressure or suction side and thus cause an earlystall. A first deflector element is placed on the suction side whichaffects the maximum lift coefficient. A second deflector element isplaced on the pressure side which affects the minimum lift coefficient.The first and second deflector elements are placed at a chord length of5% to 15% from the leading edge. This solution requires two separatedeflector elements located on opposite side surfaces in order to affectboth the maximum lift coefficient and the minimum lift coefficient.

SUMMARY OF THE INVENTION

A first object of the invention is to provide a tripping device thatreduces the maximum lift coefficient as well as increases the minimumlift coefficient.

A second object of the invention is to provide a tripping device thatdoes not reduce the aerodynamic performance within the normal operatingrange.

A third of the invention is to provide a method of reducing the loads ofa wind turbine blade that reduces the maximum lift coefficient and atthe same time increases the minimum lift coefficient.

Another object of the invention is to provide a method of reducing theloads of a wind turbine blade that can be combined with the effect ofvortex generator units.

The objects of the invention are achieved by a wind turbine comprising awind turbine tower, a nacelle arranged on top of the wind turbine tower,a rotatable rotor with at least two wind turbine blades arrangedrelative to the nacelle, wherein each of the at least two wind turbineblades comprises a tip end, a blade root and an aerodynamic profile,wherein the aerodynamic profile defines a leading edge, a trailing edge,a first side surface which geometrically defines a pressure side (alsoknown as a lower surface) and a second side surface which defines asuction side (also known as an upper surface), wherein a chord extendsfrom the leading edge to the trailing edge and has a chord with anormalized length of 100%, wherein at least one first tripping device isarranged on the pressure side on at least one of the wind turbineblades, the at least one first tripping device is placed at apredetermined position relative to the leading edge, and wherein the atleast one first tripping device is configured to passively reduce amaximum lift coefficient when said at least one of the wind turbineblades has a positive AoA outside a normal operating angle-of-attack(AoA) range and to passively increase a minimum lift coefficient whensaid at least one of the wind turbine blades has a negative AoA outsidethe normal operating AoA range.

The maximum lift coefficient, C_(L-max), is defined as the maximumpositive lift coefficient as function of a positive AoA. The minimumlift coefficient is defined as the minimum negative lift coefficient,C_(L-min), as function of a negative AoA. The AoA is determined based onthe pitch angle of the wind turbine blade, the rotor speed, and the windspeed of the wind turbine. During normal operating conditions of thewind turbine blade, the lift coefficient varies with the AoA between theminimum and maximum lift coefficients.

Conventional stall strips, as in U.S. Pat. No. 8,602,739 B2 and U.S.Pat. No. 8,083,491 B2, are designed to only alter the minimum liftcoefficient of the wind turbine blade and are thus only designed tofunction at negative AoAs. Similarly, conventional flaps or slats aredesigned to only increase the maximum lift coefficient and thecorresponding AoA thus these devices are only designed to function atpositive AoAs. Other conventional stall devices, as in U.S. Pat. No.8,192,161, has a first stall element altering the maximum liftcoefficient and a second stall element altering the minimum liftcoefficient.

The present tripping device is advantageously configured to function atboth positive and negative AoAs and thus alters both the minimum andmaximum lift coefficients. This reduces the extreme loads occurring whenthe wind turbine blade is operated outside the normal operating range,e.g., due to extreme wind speeds, wind gusts, wind shares or windturbulence.

The present tripping device reduces the maximum lift coefficient whileat the same time increasing the minimum lift coefficient so that bothvalues are moved towards each other which in turn reduces the range ofthe lift coefficient of the wind turbine blade. This reduces the fatigueloads occurring as AoA changes from a positive AoA to a negative AoA, orvice versa, when the wind turbine blade is operated outside the normaloperating range.

According to one embodiment, said at least one tripping device isfurther configured to transform an airflow passing over the at least onetripping device from a laminar airflow into a turbulent airflow when theAoA of said at least one of the wind turbine blades is outside thenormal operating AoA range in one direction.

As the wind passes the wind turbine blade, the airflow thereof developsa plurality of boundary layers over the pressure and suction sides inthe chord-wise direction. The incoming airflow forms a so calledstagnation point or region in the leading edge area of the wind turbineblade in which the wind speed is essentially zero. The airflow thencontinues along the pressure and suction sides respectively as a laminarairflow and thus laminar boundary layers. A similar stagnation point orregion may also be found beyond the trailing edge area.

Conventional stall strips, as in U.S. Pat. No. 8,602,739 B2 and U.S.Pat. No. 8,083,491 B2, are designed to cause the passing airflow toseparate along the side surface in the chord-wise direction from thestall strip towards the trailing edge, and thus, separate the boundarylayers at an earlier point than without such stall devices. This causesthe wind turbine blade to stall earlier and in turn reduces the liftearlier than compared to wind turbine blades without such stall devices.These stall strips define a so called separation point or region atwhich the airflow starts to form a circulation zone.

The tripping device advantageously transforms the laminar airflow andthus the laminar boundary layers into a turbulent airflow and thusturbulent boundary layers. The tripping device defines a transitionpoint or region at which the airflow changes from one condition toanother condition. This turbulent airflow then continues along the sidesurface into the possible separation point or region at which theairflow separates as described above. This in turn may also reduce theamount of ice, dust or other particles accumulating on the side surfacesdue to the turbulent airflow.

According to one embodiment, the position of the at least one firsttripping device is determined according to a distance along the chordand projected towards the pressure side, where the at least one trippingdevice is positioned at a distance of 0% to 1% measured along the chordfrom the leading edge, preferably at a distance of 0.2% to 0.6% from theleading edge.

The transition point/region is located towards the leading edgestagnation point/region and the separation point/region is locatedtowards the trailing edge stagnation point/region. This in turn meansthat the present tripping device is advantageously positioned closer tothe leading edge compared to the conventional stall strips, as in U.S.Pat. No. 8,083,491.

The wind turbine blade has a total normalised chord length of 100%measured from leading edge to trailing edge. The position of thetripping device is determined by projecting its position onto the chordand measuring the distance from the intersection point to the leadingedge. Tests have demonstrated that the tripping device should bepositioned at a distance of 0% to 1%, preferably 0.2% to 0.6%, from theleading edge for optimal effect. Whereas conventional stall strips arepreferably positioned at a distance of 5% to 20% for optimal effect. Byplacing the tripping device closer to the leading edge, an earlierreduction of the maximum and minimum lift coefficients can be achieved.

The abovementioned distance is measured relative to a front edge of thetripping device facing the leading edge of the wind turbine blade.Alternatively, the distance may be measured relative to a centrallongitudinal line of the tripping device or a back edge of the trippingdevice facing the trailing edge of the wind turbine blade. In this casethe installation of the tripping device has to be adjusted accordingly,i.e. the numeric value of the above-mentioned distance has to beproportionally larger than when measuring relative to the front edge ofthe tripping device.

The chord length of the wind turbine blade at a predeterminedlongitudinal point, e.g., the point defining the maximum chord length,may be used to determine the chord-wise position of the tripping device.In example, the tripping device may be positioned at a wind turbineblade in the section of the blade where the chord length is between 600mm and less.

According to one embodiment, the at least one first tripping device orthe at least one second tripping device has a height of 0.05 mm to 1 mm,preferably a height of 0.2 mm to 0.5 mm.

The tripping device has a longitudinal length extending parallel to thelongitudinal length of the wind turbine, a width extending in thechord-wise direction, and a height extending outwards from therespective side surface. These dimensions of the tripping device andoptionally also the position are optimized relative to the shape andsize of the respective wind turbine blade.

The tests have further demonstrated that the tripping device should havea height between 0.05 mm and 1 mm, preferably between 0.2 mm and 0.5 mm,for optimal effect. This enables the tripping device to alter theconditions of the airflow without causing the airflow to separate.

Conventional stall strips need to be higher in order to separate theairflow, e.g., more than twice the height of the tripping device.However, such a device will increase the drag of the wind turbine bladeand thus reduce the lift.

According to one embodiment, the at least one first tripping device orthe at least one second tripping device has a width of 0.5 mm to 5 mm,preferably a width of 1 mm to 3 mm.

The tests have further demonstrated that the tripping device should havea width between 0.5 mm and 5 mm, preferably between 1 mm and 3 mm, foroptimal effect. This also enables the tripping device to alter theconditions of the airflow without causing the airflow to separate.

Conventional stall strips normally also need to be thicker due to theincreased height, e.g., twice the height of the tripping device. Suchstall strips may also be subject to greater forces than the trippingdevice and thus must be designed accordingly. If such a stall strip wereplaced closer to the leading edge, then it will have a negative effecton the aerodynamic performance of the wind turbine blade during normaloperating conditions. The present invention is designed to have no oralmost no adverse effect on the aerodynamic performance of the windturbine blade during normal operating conditions.

The test results indicate that the maximum lift coefficient can bereduced by about 10% compared to a wind turbine blade without such atripping device. The test results also indicate that the minimum liftcoefficient can be increased by about 16% compared to a wind turbineblade without such a tripping device. This, in turn, means that therange of the lift coefficient can be reduced by more than 20%.

According to one embodiment, at least one second tripping device isfurther arranged on the pressure side of the at least one wind turbineblade, the at least one second tripping device is placed at anotherpredetermined position relative to the leading edge, wherein the atleast one second tripping device is arranged relative to the at leastone first tripping device.

Two or more tripping devices may be arranged relative to each other onthe pressure side of the wind turbine blade, preferably in thechord-wise direction. The tripping devices may all be configured toalter the maximum and minimum lift coefficients. Alternatively, a firstset of tripping devices may be configured to alter the maximum liftcoefficient and a second set of tripping devices may be configured toalter the minimum lift coefficient. This allows the dimensions andoptionally the position of the respective tripping device to beoptimized individually or in pairs relative to the shape and size of therespective wind turbine blade.

It has not previously been disclosed to use two or more devices on thesame wind turbine blade cross section. The conventional stall strips asdescribed in U.S. Pat. No. 8,083,491 B2 and U.S. Pat. No. 8,602,739 B2all teach the use of a single device in a given cross section.

According to a special embodiment, the position of the at least onesecond tripping device is determined according to a distance along thechord and projected towards the pressure side, where the at least onesecond tripping device is positioned at a distance of 1% to 3% measuredalong the chord from the leading edge, preferably at a distance of 1.5%to 2.5% from the leading edge.

A first tripping device may be positioned as described above while atleast one second tripping device is positioned between the firsttripping device and the trailing edge. The second tripping device may bealigned with the first tripping device in the chord-wise direction. Inexample, the first tripping device may be positioned at a first distanceof 0% to 1%, preferably 0.25% to 0.5%, while the second tripping devicemay be positioned at a second distance of 1% to 3%, preferably 1.5% to2.5%, most preferably 2% along the chord. Said first tripping device maybe optimized to alter the maximum lift coefficient and said secondtripping device may be optimized to alter the minimum lift coefficient.

According to another special embodiment, each of the at least two windturbine blades has a longitudinal length extending from the blade rootto the tip end, wherein the at least one second tripping device isoffset relative to the at least one first tripping device in thelongitudinal direction.

The above-mentioned first and second tripping devices may further beoffset relative to each other in the longitudinal direction. The firsttripping device may be positioned at a first longitudinal length and thesecond tripping device may be positioned at a second longitudinallength. The first and second longitudinal lengths are measured from theblade root of the wind turbine blade. Alternatively, a third trippingdevice may be positioned relative to the first and second trippingdevices in the chord-wise and/or longitudinal direction.

For example, the first tripping device may be positioned at a firstdistance of 0.1% to 0.3%, preferably 0.15% to 0.25%, while the secondtripping device may be positioned at a second distance of 0.3% to 0.5%,preferably 0.35% to 0.45%—all measured along the chord.

The first and second tripping devices may be partly offset so that onlya part of the respective devices are aligned with each other. The firstand second tripping devices may be fully offset so that the respectivedevices have opposite facing ends. Said opposite facing ends may bealigned with each other or may define a gap between the first and secondtripping device. This further enables the placement of the respectivetripping device to be optimized according to the aerodynamic profile anddimensions of the respective wind turbine blade.

According to one embodiment, one of the at least one first trippingdevice and the at least one second tripping device have a rectangularshape in the chord-wise direction. The tripping device, i.e., the firstor second tripping device, may be made of plastics, metal, wood,composites, a fibrous reinforced material or another suitable material.The tripping device may have a quadrilateral, triangular, circularsectional cross-sectional shape when seen in the chord-wise direction.The quadrilateral shape includes a squared, rectangular or trapeziumshape. The triangular shape includes an isosceles or scalene triangularshape. Said circular sectional shape includes a semi-circular orquarter-circular shape. The tripping device may also have a zig-zagshape when seen in the longitudinal direction.

In example, the tripping device preferably has a rectangular shape whichis more resilient to erosions than the triangular shape of theconventional stall strips as described in U.S. Pat. No. 8,602,739 B2.

According to one embodiment, each of the at least two wind turbineblades has a longitudinal length of 100% extending from the blade rootto the tip end, wherein one of the at least one first tripping deviceand the at least one second tripping device are positioned along said atleast one of the wind turbine blades from a distance of at least 33%measured along the longitudinal length from the blade root, preferablyfrom a distance of at least 66% measured from the blade root, where theat least one tripping strip extends towards the tip end or the vicinityof the tip end of the wind turbine blade.

The wind turbine blade has a total normalized longitudinal length of100% measured from blade root to tip end. The tripping device isadvantageously placed towards the tip end of the wind turbine blade,preferably at a distance of at least 33% measured from the blade rootwhen the position of the tripping device is projected onto thelongitudinal length. The tripping device extends in the longitudinaldirection and has a predetermined longitudinal length. The conventionalstall strips are preferably placed closer to the blade root than thetripping device. This suggests that the stall strips are more subjectedto different wind speeds and loads than the tripping device.

The tripping device, i.e., the first or second tripping device, may forma single continuous device or two, three or more sub-devices placed in aconsecutive order. Alternatively, one or more of these sub-devices maybe offset relative to the remaining devices in the chord-wise direction.The use of multiple sub-devices allows for easier handling andinstallation. The tripping device or sub-devices may be formed as tapesor stripes extending in the longitudinal direction.

According to one embodiment, at least one vortex generator unit isarranged on the suction side of said at least one of the wind turbineblades, the at least one vortex generator unit is placed at apredetermined position relative to the leading edge.

The tripping devices may advantageously be combined with one or morevortex generators (VG) units arranged on the wind turbine blade,preferably on the suction side. The VG-unit may be configured toincrease the lift coefficient of the wind turbine blade. The VG-unit mayfurther be configured to increase the corresponding AoA of the maximumlift coefficient. This allows the VG-unit to compensate for the liftloss due to surface roughness, e.g., caused by wear or dirt, ice orother contaminations.

An object of the invention is also achieved by a method of reducingloads in a wind turbine comprising a wind turbine tower, a nacellearranged on top of the wind turbine tower, a rotatable rotor with atleast two wind turbine blades arranged relative to the nacelle, whereineach of the at least two wind turbine blades comprises a tip end, ablade root and an aerodynamic profile, wherein the aerodynamic profiledefines a leading edge, a trailing edge, a first side surface whichdefines a pressure side and a second side surface which defines asuction side, wherein a chord extends from the leading edge to trailingedge and has a chord length of 100%, wherein the method comprises thesteps of:

arranging at least one tripping device at a predetermined positionrelative to the leading edge on the pressure side of at least one of thewind turbine blades,

operating said at least one of the wind turbine blades within a normaloperating AoA range,

passively reducing a maximum lift coefficient via said at least onetripping device when said at least one of the wind turbine blades has apositive AoA outside the normal operating AoA range, and passivelyincreasing a minimum lift coefficient via said at least one trippingdevice when said at least one of the wind turbine blades has a negativeAoA outside the normal operating AoA range.

The present configuration enables the tripping device to be activated,i.e., to function, outside the normal operating conditions of the windturbine blade so that it does not affect, i.e., reduces the lift, duringthe normal operation of the wind turbine blade. Said normal operatingconditions may be defined by the normal pitching range of wind turbineblade or the normal operating AoA range. The range may be defined by afirst and a second limit value, e.g., a first and second pitch angle ora first and second AoA value. In example, the normal pitch range may be3 degrees to 10 degrees, e.g., 8 degrees.

Unlike conventional stall strips or other stall introducing or liftenhancing devices arranged on the wind turbine blade, the trippingdevice is activated at both positive and negative AoAs outside thenormal operating range. This in turn means that the maximum and minimumlift coefficients are reduced which in turn reduce the extreme loads andfatigue loads of the wind turbine blade and other components of the windturbine.

In normal operation, the wind turbine is pitched within the normal pitchrange to maximize the power output. The lift of wind turbine accordinglycycles along the lift-to-AoA curve between the maximum and minimum liftcoefficients. In this normal mode, the tripping device is not activated.

According to one embodiment, said at least one tripping devicetransforms an airflow passing over said at least one tripping devicefrom a laminar airflow into a turbulent airflow when the AoA of said atleast one of the wind turbine blades is outside the normal operating AoArange in one direction.

In an extreme event, e.g., due to high wind speeds, wind gusts, windshears, wind turbulence or other extreme conditions, the wind turbineblade is pitched out of the normal operating range or the AoA changes toa value outside the normal operating range, thereby activating thetripping device. As the AoA exceeds the limit value of the normaloperating range in positive or negative direction, the tripping devicecauses an earlier transformation of the passing airflow and, thus,reduces the extreme loads. Once the extreme event is over, the windturbine blade is pitched back into the normal operating range or the AoAchanges back to a value inside the normal operating range, therebydeactivating the tripping device.

According to one embodiment, the step of arranging the at least onetripping device comprises one of:

attaching the at least one tripping device to the at least one of thewind turbine blades after manufacture of the at least one of the windturbine blades, or

providing the at least one tripping device, e.g., integrating the atleast one tripping device into the side surface, during manufacture ofthe at least one of the wind turbine blades.

The tripping device may be formed as part of the wind turbine bladeduring the manufacture, e.g., by grooves formed in a mold which are thenfilled with fibrous material during the layup and subsequent cured withresin. Alternatively, the tripping device may be formed during thefinishing step of the manufacturing process. This solution is sensitiveto tolerances during layup or during the finishing step of themanufacturing process.

One way to solve this problem is to form the tripping device as aseparate device which then is fixed to the side surface after themanufacturing process. This allows for an easy removal and replacementif the tripping device gets damaged. The tripping device may be adheredto the side surface means using an adhesive layer. The tripping devicemay be shaped in a 3M-tape (also known as helicopter tape) or aPolyurethane (PU) tape and then applied to the wind turbine blade. Thetripping device may also be glued to the side surface using glue appliedto the bottom surface of the tripping device and/or the side surface ofthe wind turbine blade. Alternatively, the tripping devices may beattached by using fastening elements such as bolts or rivets.

According to one embodiment, the at least one tripping device ispositioned on the at least one of the wind turbine blades using aninstallation tool, wherein said installation tool comprises means forreceiving the at least one tripping device and means for aligning the atleast one tripping device relative to at least one reference point.

The tripping device may be correctly positioned on the respective sidesurface using an installation tool. The installation tool may compriseone or more openings for receiving and holding one or more trippingdevices. The installation tool may further comprise one or morepositioning means for placement of the tool on a part, e.g., the sidesurface and/or leading edge, of the wind turbine blade. Said positioningmeans may be a support bracket or a foot, where the bracket or foot areshaped to conform to the contours of a specific part of the wind turbineblade, and alternatively, said support bracket or foot may be adjustableso that they can be adjusted to follow the contours of the wind turbineblade.

The installation tool may further comprise aligning means for aligningthe tripping device relative to one or more reference points of the windturbine blade. Said aligning means may be a measuring unit, a lightsource such as a laser unit, a fixed unit having a predetermined sizeand shape or other suitable means for aligning the tripping device. Thealigning means may be adjustable so that the position of the trippingdevice can be moved into the correct position. The reference point maybe a marking of the wind turbine blade, the leading or trailing edge,the tip end or another suitable reference point.

For example, the tip end and trailing edge may be used as referencepoints as they are easy to define contrary to the leading edge, which ismore difficult to define. The positioning means may further comprise oneor more openings for positioning the installation tool relative to oneor more tripping devices already installed. The installation tool may beoperated manually or via a controller connected to the installationtool. This allows for an easier installation process of the trippingdevice relative to the reference points and, in turn, a more preciseplacement of the tripping device on new wind turbine blades as well asold wind turbine blades.

The installation tool may further comprise means for fixing the trippingdevice to the wind turbine blade. Said fixing means may be a unit forapplying glue or an adhesive to the bottom surface of the trippingdevice. The fixing means may also be configured to apply pressure andoptionally heat to the tripping device when it has been placed on thewind turbine blade. This ensures a good fixture between the trippingdevice and the wind turbine.

Once the installation process is completed, the installation tool may beremoved from the wind turbine blade and the tripping devices.

The invention is described by example only and with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment of a wind turbine;

FIG. 2 shows the airflow around the wind turbine blade without a stalldevice;

FIG. 3 shows the airflow around the wind turbine blade outfitted with astall device;

FIG. 4 shows a first embodiment of a tripping device according to theinvention;

FIG. 5 shows a second embodiment of the tripping device and a stalldelaying device according to the invention;

FIG. 6 shows a third embodiment of the tripping device and the stalldelaying device according to the invention;

FIG. 7a shows an embodiment of a first tripping device installed alongapproximately the outer 25% of a blade;

FIG. 7b shows an embodiment of a first tripping device installed alongapproximately the outer 60% of a blade;

FIG. 7c shows embodiments of a first and second tripping deviceaccording to the invention;

FIG. 8 shows the airflow around the wind turbine blade without thetripping device;

FIG. 9 shows the airflow around the wind turbine blade outfitted withthe tripping device;

FIG. 10 shows a first graph of the lift coefficient relative to theangle-of-attack;

FIG. 11 shows a first graph of the lift coefficient relative to the dragcoefficient;

FIG. 12 shows a second graph of the lift coefficient relative to theangle-of-attack;

FIG. 13 shows a second graph of the lift coefficient relative to thedrag coefficient; and

FIG. 14 shows a graph of the maximum lift coefficient relative to theposition of the tripping device.

DETAILED DESCRIPTION OF THE INVENTION

In the following text, the figures will be described one by one and thedifferent parts and positions seen in the figures will be numbered withthe same numbers in the different figures. Not all parts and positionsindicated in a specific figure will necessarily be discussed togetherwith that figure.

REFERENCE LIST

-   1. Wind turbine-   2. Wind turbine tower-   3. Nacelle-   4. Wind turbine blades-   5. Pitch mechanism-   6. Tip end-   7. Blade root-   8. Leading edge-   9. Trailing edge-   10. Pressure side-   11. Suction side-   12. Airflow-   13. Stall device-   14. Tripping device, first tripping device-   15. Stall delaying device-   16. Second tripping device-   17. Maximum lift coefficient-   18. Minimum lift coefficient-   19. Normal operating range-   20. Position of tripping device-   21. Chord

FIG. 1 shows an exemplary embodiment of a wind turbine 1 comprising awind turbine tower 2. A nacelle 3 is arranged on top of the wind turbinetower 2 and connected to the wind turbine tower 2 via a yaw mechanism(not shown). A rotor comprising at least two wind turbine blades 4, herethree blades are shown, is rotatably connected to a drive train arrangedinside the nacelle 3 via a rotation shaft. The wind turbine blade 4 isrotatably connected to a hub via a pitch mechanism 5 controlled by apitch control system.

Each wind turbine blade 4 has a tip end 6, a blade root 7 and a bodyhaving an aerodynamic profile which defines a leading edge 8 and atrailing edge 9. The side surfaces of the aerodynamic shaped body definethe pressure side 10 and the suction side 11 respectively.

FIG. 2 shows the airflow around the wind turbine blade 4 with no stalldevices provided on the pressure side 10. The incoming wind hits thewind turbine blade at an AoA measured relative to the chord extendingfrom the leading edge 8 to the trailing edge 9. The passing airflow 12develops a plurality of boundary layers over the respective pressure andsuction sides 10, 11 as illustrated in FIGS. 2 and 3.

FIG. 3 shows the passing airflow 12 around the wind turbine blade 4outfitted with a conventional stall device 13. The stall device 13 isdesigned to separate the passing airflow 12 along the suction side 11 asillustrated in FIG. 3 and thereby causing the wind turbine blade 4 tostall earlier compared to the wind turbine blade 4 of FIG. 2.

FIG. 4 shows a first embodiment of a tripping device 14 arranged on thepressure side 10 of the wind turbine blade 4. The tripping device 14 ispositioned relative to the leading edge 8 and has a longitudinal length,a transverse width and a height extending outwards from the pressureside 10.

The tripping device 14 is preferably positioned at a distance of 0% to1% measured along the chord 21 (indicated by the dashed line) from theleading edge 8. As illustrated, the tripping device 14 has a rectangularcross-sectional profile, however, other cross-sectional profiles may beused depending on the respective aerodynamic profile and dimensions ofthe wind turbine blade 4. The dimension of the tripping device 14 isdesigned for transforming the passing airflow 12 from a laminarcondition to a turbulent condition. This, in turn, reduces the maximumlift coefficient and also increases the minimum lift coefficient of thewind turbine blade 4. The tripping device 14 has a width between 0.5 mmand 5 mm and a height between 0.05 mm to 1 mm for optimal effect on themaximum and minimum lift coefficients.

FIG. 5 shows a second embodiment wherein a stall delaying device 15 inthe form of a VG-unit is arranged on the suction side 11 of the windturbine blade 4 shown in FIG. 4. The stall delaying device 15 isdesigned for a different purpose, i.e., delaying stall, than thetripping device 14. Thus, the cross-sectional profile and dimensions ofthe stall delaying device 15 differ from those of the tripping device14. The profile of the stall delaying device 15 causes the overall liftcoefficient of the wind turbine blade 4 to increase which in turncompensates for the lift loss due to an increasing surface roughness ofthe wind turbine blade 4. The dimensions of the stall delaying deviceare known and will not be described in further details.

The positions of the tripping device 14 and the stall delaying device 15is projected onto the chord 21 (dashed line) and measured along thechord 21 relative to the leading edge 8. As illustrated, the stalldelaying device 15 is placed at a distance located closer to thetrailing edge 9 compared to that of the tripping device 14.

FIG. 6 shows a third embodiment wherein a second tripping device 16 isfurther arranged on the pressure side 10 of the wind turbine blade shownin FIG. 5. The first tripping device 14 is positioned at a firstdistance from the leading edge 8 while the second tripping device 16 ispositioned at a second distance from the leading edge 8. As illustrated,the second tripping device 16 is positioned between the first trippingdevice 14 and the trailing edge 9. The first and second distances arehere measured along the chord and projected as illustrated with thedashed line normal to the chord 21.

The cross-sectional shape and dimensions of the first tripping device 14is optimized to reduce the maximum lift coefficient. The cross-sectionalshape and dimensions of the second tripping device 16 is optimized toincrease the minimum lift coefficient. The first and second trippingdevices 14, 16 of FIG. 6 are aligned relative to each other in thechord-wise direction.

FIG. 7a shows an embodiment of a first tripping device 14 installedalong approximately the outer 25% of a blade 4 seen from the pressureside 10. In this embodiment, the stall delaying device 15 may beomitted.

FIG. 7b shows an embodiment of a first tripping device 14 installedalong approximately the outer 60% of a blade 4 seen from the pressureside 10. In this embodiment, the stall delaying device 15 may beomitted.

FIG. 7c shows embodiments of a first and second tripping deviceaccording to the invention, where the second tripping device 16 isoffset relative to the first tripping device 14 in a longitudinaldirection towards the tip end 6. The wind turbine blade 4 has alongitudinal length extending from the blade root 7 to the tip end 6.The first tripping device 14 is placed at a first distance betweenapproximately 25% to 65% measured along the longitudinal length from theblade root 7. The second tripping device 16 is placed at a seconddistance of approximately 65% to 100% measured along the longitudinallength from the blade root 7. The offset between the first and secondtripping devices 14, 16 may be determined according to the aerodynamicprofile and dimensions of the respective wind turbine blade 4. Here, theoffset is approximately 0%.

The first and second tripping devices 14, 16 is in FIG. 7c illustratedas being offset relative to each other in the chord-wise direction.However, the second tripping device 16 may also be aligned in thechord-wise direction with the first tripping device 14 so that bothtripping devices are placed at the same distance from the leading edge8.

FIG. 8 shows the passing airflow 12 around the wind turbine blade 4without the tripping device 14, 16. As illustrated, the incoming windinitially forms a stagnation point, S, near the leading edge 8.

The airflow 12 then passes along the pressure and suction sides 10, 11towards the trailing edge 9. The airflow 12′ on the pressure side 10then passes a transition point, TP1, where the airflow 12′ transformsfrom a laminar airflow, LF, to a turbulent airflow, TF. Likewise, theairflow 12″ on the suction side 11 then passes a transition point, TP2,where the airflow 12″ transforms from a laminar airflow, LF, to aturbulent airflow, TF.

The airflow then further passes towards the trailing edge 9 and finallyseparates from the respective pressure and suction sides 10, 11 at aseparation point (not shown).

FIG. 9 shows the airflow 12 around the wind turbine blade 4 outfittedwith the tripping device 14. As illustrated, the tripping device 14forms a transition point, TP4, where the airflow 12″ transforms from alaminar airflow, LF, to a turbulent airflow, TF. This causes an earliertransformation of the airflow 12 and thus reduces the effect of thelaminar airflow compared to the wind turbine blade of FIG. 8.

FIG. 10 shows a first graph of the lift coefficient, C_(L), relative tothe angle-of-attack, AoA. A first curve (solid line) shows the liftcoefficient of the wind turbine blade 4 where no tripping device 14 isprovided on the pressure side 10. A second curve (dashed line) shows thelift coefficient where a tripping device 14 is provided on the pressureside 10 at a distance of 0.25%. A third curve (dotted line) shows thelift coefficient where a tripping device 14 is provided on the pressureside 10 at a distance of 0.5%.

As illustrated by the second and third curves, the maximum liftcoefficient 17 is reduced towards the minimum lift coefficient comparedto that of the first curve. Likewise, the minimum lift coefficient 18 isincreased towards the maximum lift coefficient compared to that of thefirst curve. This reduces the extreme loads occurring when the windturbine blade 4 is pitched outside the normal pitch range.

FIG. 11 shows a first graph of the lift coefficient, C_(L), relative tothe drag coefficient, C_(d). A first curve (solid line) shows the liftcoefficient of the wind turbine blade 4 where no tripping device 14 isprovided on the pressure side 10. A second curve (dashed line) shows thelift coefficient where a tripping device 14 is provided on the pressureside 10 at a distance of 0.25%. A third curve (dotted line) shows thelift coefficient where a tripping device 14 is provided on the pressureside 10 at a distance of 0.5%.

As illustrated by the second and third curves, the tripping device 14does not adversely affect the lift coefficient in the normal operatingrange 19. The tripping device 14 reduces the range between the maximumlift coefficient 17 and the minimum lift coefficient 18. This alsoreduces the fatigue loads occurring when the wind turbine blade 4 ispitched within the normal pitch range.

FIG. 12 shows a second graph of the lift coefficient, C_(L), relative tothe angle-of-attack, AoA. This graph differs from the first graph ofFIG. 10 by both a stall delaying device 15 and a tripping device beingprovided on the wind turbine blade 4.

As illustrated by the second and third curves (dashed and dotted lines),the maximum lift coefficient 17 is increased compared to that of thefirst curve of FIG. 10. Likewise, the AoA corresponding to the maximumlift coefficient 17 is increased compared to that of the first curve ofFIG. 10.

FIG. 13 shows a second graph of the lift coefficient, C_(L), relative tothe drag coefficient, C_(d). This graph differs from the first graph ofFIG. 11 by both a stall delaying device 15 and a tripping device beingprovided on the wind turbine blade 4.

As illustrated by the second curve (dashed line), the lift coefficientis increased compared to the third curve of FIG. 11.

FIG. 14 shows a graph of the maximum lift coefficient 17, C_(L-max),relative to the position 20, x/c, of the tripping device 14. Theposition 20 is defined as the distance from the leading edge 8 along thechord measured in percentage of the total normalised chord length of thewind turbine blade 4.

As illustrated, the optimal effect of the tripping device 14 is obtainedif the tripping device 14 is positioned at a distance between 0% and 1%,preferably between 0.2% and 0.6%. If the tripping device 4 is positionedat a distance greater than 1%, the tripping device has substantially noeffect on the maximum lift coefficient 17.

What is claimed is:
 1. A wind turbine comprising: a wind turbine tower,a nacelle arranged on top of the wind turbine tower, a rotatable rotorwith at least two wind turbine blades connected to the nacelle, whereineach of the at least two wind turbine blades comprises a tip end, ablade root and an aerodynamic profile, wherein the aerodynamic profiledefines a leading edge, a trailing edge, a first side surface whichdefines a pressure side, and a second side surface which defines asuction side, wherein a chord extends from the leading edge to thetrailing edge and has a chord length of 100%, wherein at least one firsttripping device is arranged on the pressure side on at least one of thewind turbine blades, the at least one first tripping device is placed ata predetermined position relative to the leading edge, and wherein theat least one first tripping device is configured to passively reduce amaximum lift coefficient when said at least one of the wind turbineblades has a positive AoA outside a normal operating AoA range and topassively increase a minimum lift coefficient when said at least one ofthe wind turbine blades has a negative AoA outside the normal operatingAoA range.
 2. The wind turbine according to claim 1, wherein said atleast one first tripping device is further configured to transform anairflow passing over the at least one first tripping device from alaminar airflow into a turbulent airflow when the AoA of said at leastone of the wind turbine blades is outside the normal operating AoA rangein one direction.
 3. The wind turbine according to claim 1, wherein thepredetermined position of the at least one first tripping device isdetermined according to a distance along the chord and projected towardsthe pressure side, wherein the at least one first tripping device ispositioned at a distance of 0% to 1% measured along the chord from theleading edge.
 4. The wind turbine according to claim 3, wherein the atleast one first tripping device is positioned at a distance of 0.2% to0.6% from the leading edge.
 5. The wind turbine according to claim 1,wherein at least one second tripping device is further arranged on thepressure side of the at least one of the wind turbine blades, the atleast one second tripping device is placed at another predeterminedposition relative to the leading edge, wherein the at least one secondtripping device is arranged at a distance from the at least one firsttripping device.
 6. The wind turbine according to claim 5, wherein thepredetermined position of the at least one second tripping device isdetermined according to a distance along the chord and projected towardsthe pressure side, where the at least one second tripping device ispositioned at a distance of 1% to 3% measured along the chord from theleading edge.
 7. The wind turbine according to claim 6, wherein the atleast one second tripping device is positioned at a distance of 1.5% to2.5% from the leading edge.
 8. The wind turbine according to claim 5,wherein each of the at least two wind turbine blades has a longitudinallength extending from the blade root to the tip end, wherein the atleast one second tripping device is offset relative to the at least onefirst tripping device in the longitudinal direction.
 9. The wind turbineaccording to claim 5, wherein the at least one first tripping device orthe at least one second tripping device has a height of 0.05 mm to 1 mm.10. The wind turbine according to claim 9, wherein the at least onefirst tripping device or the at least one second tripping device has aheight of 0.2 mm to 0.5 mm.
 11. The wind turbine according to claim 5,the at least one first tripping device or the at least one secondtripping device has a width of 0.5 mm to 5 mm.
 12. The wind turbineaccording to claim 11, wherein the at least one first tripping device orthe at least one second tripping device has a width of 1 mm to 3 mm. 13.The wind turbine according to claim 5, wherein one of the at least onefirst tripping device and the at least one second tripping device have arectangular shape in the chord-wise direction.
 14. The wind turbineaccording to claim 5, wherein each of the at least two wind turbineblades has a longitudinal length of 100% extending from the blade rootto the tip end, wherein one of the at least one first tripping deviceand the at least one second tripping device is positioned at a distanceof at least 33% measured along the longitudinal length from the bladeroot.
 15. The wind turbine according to claim 14, wherein one of the atleast one first tripping device and the at least one second trippingdevice is positioned at a distance of at least 66% from the blade root.16. The wind turbine according to claim 1, wherein at least one vortexgenerator unit is arranged on the suction side of said at least one ofthe wind turbine blades, the at least one vortex generator unit isplaced at a predetermined position relative to the leading edge.
 17. Amethod of reducing loads in a wind turbine, comprising a wind turbinetower, a nacelle arranged on top of the wind turbine tower, a rotatablerotor with at least two wind turbine blades arranged relative to thenacelle, wherein each of the at least two wind turbine blades comprisesa tip end, a blade root and an aerodynamic profile, wherein theaerodynamic profile defines a leading edge, a trailing edge, a firstside surface which defines a pressure side and a second side surfacewhich defines a suction side, wherein a chord extends from the leadingedge to the trailing edge and has a chord length of 100%, wherein themethod comprising the steps of: arranging at least one tripping deviceat a predetermined position relative to the leading edge on the pressureside of at least one of the wind turbine blades, operating said at leastone of the wind turbine blades within a normal operating AoA range,passively reducing a maximum lift coefficient via said at least onetripping device when said at least one of the wind turbine blades has apositive AoA outside the normal operating AoA range, and passivelyincreasing a minimum lift coefficient via said at least one trippingdevice when said at least one of the wind turbine blades has a negativeAoA outside the normal operating AoA range.
 18. The method according toclaim 17, wherein said at least one tripping device transforms anairflow passing over said at least one tripping device from a laminarairflow into a turbulent airflow when the AoA of said at least one ofthe wind turbine blades is outside the normal operating AoA range in onedirection.
 19. The method according to claim 17, wherein said step ofarranging at least one tripping device comprises one of: attaching theleast one tripping device to said at least one of the wind turbineblades after manufacture of the at least one of the wind turbine blades,or providing the at least one tripping device during manufacturing ofthe at least one of the wind turbine blades.
 20. The method according toclaim 17, wherein the at least one tripping device is positioned on theat least one of the wind turbine blades using an installation tool,wherein said installation tool comprises means for receiving the atleast one tripping device and means for aligning the at least onetripping device relative to at least one reference point.